The fusing of thermoplastic dry toner powders to receiver sheets of paper or plastic to form electrostatographic images or copies is well known in electrophotographic and dielectric recording processes. Either black and white or multicolor images can be formed, by fusing such thermoplastic toners to receiver sheets. Two types of fuser systems are commonly used for applying heat and pressure to fuse and fix the toner particles to the receiver: fuser roller systems and fuser belt systems. One problem with fuser roller systems is related to the high release temperature of the rollers, that is, the temperature at which the receiver sheet leaves the nip of the rollers. Under such conditions, the toner can act as a hot melt adhesive, causing the receiver sheet to adhere to the roller.
Fuser belt systems can reduce some of the problems encountered with fuser rollers. For example, U.S. Pat. No. 5,089,363, discloses that metal belts coated with highly cross-linked polysiloxanes produce toner images having high gloss. Such polymeric release coatings, however, have poor adhesion to the usual belt substrate materials.
Polyimide belts are highly flexible and can be more easily handled without forming kinks than metal belts. Polyimides useful as fusing belts are disclosed in U.S. Pat. No. 5,411,779, the disclosure of which is incorporated herein by reference. As disclosed in the reference, the polyimide can be prepared in tubular or belt form by coating a poly(amic acid) solution on the inner circumference of a cylinder and imidizing the poly(amic acid) to form a tubular inner layer of the polyimide resin. Following heating-drying, the polyimide belt is removed from the cylinder.
An important advantage of a polyimide as a fuser belt substrate is related to its fabrication as a seamless belt, which avoids the problem of seams that are visible in the toner image. The fact that a polyimide belt cools more rapidly than a metal belt after it leaves the heated nip of the fuser system represents another advantage of a polyimide fuser belt substrate.
Polyimides are useful materials for the fabrication of fuser belts, but they are difficult to process. U.S. Pat. No. 6,500,375, the disclosure of which is incorporated herein by reference, describes a process for forming a seamless polyimide tube using a canted doctor blade to shape a coating of a resin precursor solution on a mandrel prior to curing. As discussed in the reference, polyimides are thermosetting resins that cannot be reformed with heat and are soluble in only a limited number of relatively high-boiling solvents. Curing conditions, which entail high temperatures, are difficult to optimize and greatly affect the mechanical properties of the cured film. Finally, the cost of polyamic acid resins is high, so minimizing waste of both the starting materials and product resins is very desirable.
U.S. Pat. No. 5,433,913, the disclosure of which is incorporated herein by reference, describes a process for manufacturing a resinous tube from a heat-resistant resin such as a polyimide resin by coating a highly viscous resin precursor solution on a core and dropping a metallic die around the coating to form a tube of the resin precursor, which is cured and remove from the core.
A toner release layer can be applied to a pre-formed polyimide belt substrate by spray, dip, or ring coating. A spray-coated layer is generally not satisfactory because its uneven thickness and non-uniform surface leads to degradation of the gloss and overall quality of a fused toner image. Although dip or ring coating might be expected to provide improved coating uniformity, these methods require that the tubular substrate be firmly maintained in a cylindrical shape during coating. However, given the inevitable dimensional variability arising in the manufacture of the seamless polyimide belt substrates, it can be can difficult to achieve the required coating conditions conveniently and reliably. The apparatus of the present invention provides an effective solution to such coating problems.